Spread spectrum communication techniques are widely employed in a variety of fields one of which includes the currently known Global Positioning Systems (GPS) employed for obtaining accurate positional information of a receiving device on the basis of signals received from a constellation of GPS satellites.
The GPS arrangement is employed particularly for tracking the position of objects that move over the surface of the earth, for example, vehicles, and is also increasingly being employed in an attempt to accurately locate mobile communication devices such as mobile telephones.
The mere fact that the object to be located can comprise a vehicle such as a car, or indeed a mobile communication device being carried within the vehicle, introduces additional design constraints on the GPS receiver employed since the degree of such relative movement between the receiver and each GPS satellite will introduce a Doppler shift into the frequency of the signal received from the satellite. Since, as part of the signal processing procedure within the receiver, the frequency of the incoming signal has to be accurately determined, this can introduce a severe processing overhead. Thus, current GPS receivers tend to be designed to cater for a “worst case scenario”. For example, for a receiver designed for use in a conventional motor car, which can readily achieve surface speeds of 100 miles per hour (i.e. 45 m/s) this can lead to a potential Doppler shift of the received signal of up to 200 Hz depending on the direction of travel of the motor car relative to the incoming signal.
The signal processing within the GPS receiver is therefore designed to cater for such a potential shift in the incoming frequency. However, such functionality can prove disadvantageously restrictive when the particular movement characteristics of the vehicle do not, or are unlikely to, lead to such a frequency shift.
EP-A-0,528,530 discloses a GPS navigation system employing selective updating in which a signal indicative of the vehicle within which a receiver is sensed being stationary is derived. When generated, this signal serves to prevent further signal processing within the receiver so that previously acquired positional information can be retained for repeated use since the vehicle is known not to have moved since its position was last calculated.
Such a known system is however disadvantageously limited particularly in as much as it relies upon the accuracy of the previous signal reading obtained when the vehicle was last moving.
The present invention seeks to provide for a spread spectrum receiver and related method which can offer improvements over known systems with regard to improved signal processing within the receiver.
According to one aspect of the present invention there is provided a method of the type defined above, characterised by the step of changing the mode of processing of the incoming spread spectrum signal responsive to generation of the signal indicating the said characteristic of movement of the mobile terminal.
According to another aspect of the present invention there is provided a spread spectrum receiver of the type defined above, characterised by means for changing the mode of processing of the incoming spectrum signal in response to the signal indicating the said characteristic of movement of the mobile terminal.
In one aspect of the invention, a readily determinable movement characteristic can be employed for controlling the mode of processing of the incoming spread spectrum signal in a manner for improving the performance of the receiver and thus the GPS in which it is employed.
In still other aspects of the invention, further advantageous movement characteristic can be determined and employed in combination with a threshold velocity value so as to provide for a threshold at which the processing mode can be switched in accordance with the present invention. In still other aspects of the invention, a stationery, i.e. zero velocity, movement characteristic is identified since the greatest improvement in performance within the receiver can then be readily attained. In still other aspects of the invention, the signal tracking aspects of the processing of the incoming spread spectrum signal have greatly improved stability and weaker signals can be tracked successfully.